Mucosal epithelial cells of the small intestine were identified as a major, and possibly the only significant source of citrulline in the circulation of the rat. Contrary to earlier views, the liver was found not to release citrulline under normal conditions. The kidneys are an important source of circulating arginine, which is synthesized there from citrulline of intestinal origin. Thus the small intestine and kidneys cooperate to provide an endogenous source of arginine for use by other tissues. In other studies with isolated, perfused organs and with double-isotope labeling methods in vivo, it was shown the small intestine in the rat biosynthesizes only the smaller of two variant forms of plasma apolipoprotein B, a lipoprotein polypeptide that functions in the biogenesis and metabolism of triglyceride-rich lipoproteins. Liver, on the other hand, was found to synthesize both variants. The production ratio of the two forms of apolipoprotein B seems to be dependent on the animal's physiological state. The two apoproteins from liver are independently metabolized at different rates, consistent with the hepatic production of multiple apolipoprotein B-containing lipoproteins having different metabolic properties. Taken together, the results suggest a regulatory mechanism for determining the metabolic fate of lipids released from the liver.